1. Field of Invention
This invention relates to a process for the preparation of microporous styrene homopolymers and copolymers and to the products obtained thereby. In one aspect, the present invention is directed to microporous polymers of styrene having improved microporosity and structure. In another aspect, the present invention is directed to microporous styrene polymers and to articles comprising such polymers. In one of its more specific aspects, the present invention concerns tobacco smoke filters comprising microporous polymers of styrene made according to the process of this invention.
2. The Prior Art
The preparation of articles having microporous structures has received considerable attention in the prior art. Indeed there are several patents which disclose such articles, for various applications, and describe the methods of their preparation. For example, U.S. Pat. Nos. 2,777,824 and 3,055,297 describe the preparation of a synthetic thermoplastic polymer, such as, polyvinyl chloride, having a microreticulated structure by a method which basically comprises sintering a mixture of the resin, a plasticizer (e.g., dioctyl phthalate) and an organic liquid non-solvent for the resin (e.g., xylene) to a temperature at which the resin fuses while the system is under sufficient pressure to maintain the mixture in the liquid phase. Sintering at these conditions is continued until a microreticulated structure is formed and the non-solvent may be removed by heating and/or reducing the pressure. When used for tobacco smoke filtration, these products have not been entirely satisfactory since they frequently do not have the desired degree of uniformity and are not particularly effective in removing the undesirable constituents of the tobacco smoke stream.
U.S. Pat. No. 3,528,433 (Johnson et al.) also describes microporous structures for use in granular form as filter material for cigarettes. According to this patent, a thermoplastic synthetic resin such as polyvinyl chloride is heated together with an organic liquid which is non-solvent for the resin until the resin fuses and again heating is continued under a pressure sufficient to maintain the non-solvent in the liquid phase. The mixture may optionally contain a plasticizer for the resin which, together with the non-solvent liquid, are removed or extracted, after fusion of the resin, by contacting the fused resin with vapors of a second liquid. However, unless proper precautions are exercised, the pore structure of the resulting product will be non-uniform and hence it will not be highly effective in tobacco smoke filtration. Furthermore, when plasticizers are employed, the non-uniformity of the product will be aggrevated by the formation of an impermeable outer skin or sheath thereon.
It is noted from the foregoing description that the synthetic resin is employed as a dispersion of the resin in a non-aqueous liquid which is non-solvent for the resin. However, in a very recent patent, i.e., U.S. pat. No. 3,674,722, issued July 4, 1972 the patentees - who are also applicants of the present application - disclose a method for the preparation of microporous polyvinyl chloride which has improved microporous structure. The process disclosed therein involves the use of a vinyl chloride polymer latex, i.e., an aqueous suspension of vinyl chloride polymer in an organic liquid which is a non-solvent for the polymer. There are several differences in the manner of preparing the microporous structures of this invention from those described in this patent. These differences will become apparent from the ensuing detailed description of this invention.